Add documentation for Rails API projects

[ssunday]

As called out in #18, you can't use this gem with an API project without including ActionController::Helpers or making some dummy method like def self.helper_method(_); end in ApplicationController. It should be included in the README that this is the case.

I can raise a PR at some point, I want to verify that the gem works with the aforementioned snippet first haha.

[ssunday]

No, this is kinda problematic when running with devise as devise will load in more stuff if that method is defined. I agree with the original PR that that code should be eliminated.

[ssunday]

#37 makes this redundant